Faceplate bonding process and apparatus therefor

ABSTRACT

A method of bonding faceplates (20) to VDU screens is provided in which an adhesive material is dispensed (80) onto a surface of either the faceplate or the VDU (40), the faceplate and the VDU are brought together (90) to force the adhesive material outwards to fill the gap between the surfaces, and the adhesive material layer (41) which is formed is then cured. In a first curing step (100), curing is carried out around the edges of the faceplate to form a seal around the edges. This first step may be carried out in a dedicated positioning tool. A later curing step (120) ensures that all of the adhesive material layer is eventually cured. Positioning (90) in the dedicated tool may use specific reference points on the faceplate and the VDU rather than relying on physical spacers, and may be carried out under servo control of a system for detecting the onset of undesirable gas entrapment conditions within the adhesive layer. The invention is particularly useful for automated or semi-automated bonding of touch-plates to screens to form touch-input displays, with the avoidance of spacers and with the selective, two-stage curing enabling minimising of time in the positioning tool.

This application is a continuation-in-part of application Ser. No.08/262,728 filed on Jun. 20, 1994, now U.S. Pat. No. 5,417,791.

The present invention relates to a process for bonding a faceplate tothe display screen of a visual display unit (VDU) or monitor for acomputer system, and in particular to a process for adhering togetheropposed surfaces of a faceplate and a monitor screen using a transparentadhesive material. The process is particularly useful for bonding afaceplate to an integrated tube component (ITC) of a monitor as a stepin the manufacture of a monitor having a touch-input enabled display inwhich the faceplate has associated touch-stimuli sensors.

A monitor having a touch-sensitive display typically includes a cathoderay tube monitor and a transparent, touch-sensitive overlay which isattached to the face of the monitor. Such displays are usually part of acomputer system having a small processor. To enable a touch-sensitivedisplay to be used, computer programs are written for execution in theprocessor. The computer programs define the response of the computersystem whenever the screen is touched at particular points. Depending onthe particular program, a screen touch may cause video information to beretrieved from an external video source for display on the screen,either alone or in combination with graphics information generated bythe processor. A screen touch may also result in the display ofprocessor-generated graphics information only.

Touch-sensitive displays have been implemented using a number ofdifferent technologies for detecting touch stimuli. In one suchtechnology, a transparent overlay is placed over a CRT screen. Theoverlay contains an array of electromechanical pressure sensors whichare generally either resistive, conductive or capacitive in form. Thesensors are arranged in rows and columns spanning the CRT screen area.Each sensor therefore corresponds to a particular screen location. Asignal from one such sensor is thus indicative of a particular screenlocation. There is a problem with such sensors in that they tend topartially obscure from view whatever image is displayed on the screen.Furthermore, to limit this obscuring effect, the sensors are generallyfabricated on a fragile wafer-like substrate which is easily damagedduring assembly or use.

Another technology involves the processing of an optical signal scannedacross a CRT screen in order to determine a touch location. In general,this technology has problems associated with optical parallax.Furthermore, detection of the optical signal can be prevented by foreignbodies in the vicinity of the CRT screen.

Another technology involves an array of pressure sensitive transducersmounted around the periphery of a CRT screen. Each transducer generatesan electrical signal in response to and representative of a touchstimulus applied to the CRT screen. The relative magnitudes of thesesignals can be processed to determine the location on the CRT screen atwhich the stimulus was applied. A problem with such an arrangement isthat the transducers are sensitive to spurious vibrations of thedisplay. Furthermore, the pressure-sensitive transducers are exposed toany out-of-balance forces which may be produced within the monitorduring assembly. Such undesirable effects can be reduced by locating thearray of force transducers around the periphery of a transparent pushplate which is shaped to match the contours of, but does not makecontact with, the CRT screen. A disadvantage with this arrangementresulting from the touch plate being raised above the CRT screen is thata visually objectionable optical parallax effect is produced. Anotherdisadvantage is that internal reflections can occur between the CRTscreen and the touch-plate. Furthermore, this arrangement generallyrequires mountings for the touch-plate which are resilient enough towithstand repeated touch operations as part of a normal productlifetime, yet not so rigid as to limit movement of the touch-platerelative to the CRT screen.

EP-A-0256251 describes a touch-plate arrangement which is similar tothat described above. The touch-sensitive screen assembly comprises aframe with openings to align with mounting brackets extending from adisplay unit such as a CRT screen. The frame supports a rigidtransparent touch-plate facing the display. An array ofpressure-sensitive transducers is mounted on the touch-plate surfacefacing the display. The touch-screen assembly is held in position withdeformable members positioned at the connection between the openings onthe frame and the brackets on the display unit. The deformable membersresist shifting of the touch-plate in a direction parallel to the faceof the CRT, yet resistance to movement of the plate towards or away fromthe CRT is minimised. Thus, measurement repeatability during the normallifetime of the display apparatus is provided. However, the deformablemembers are large in relation to conventional CRT mounting screws. Inaddition, this arrangement can also be sensitive to vibrations of thedisplay and therefore complex electronic signal processing is requiredfor conditioning the signals from the transducers.

EP-A-0434314 describes a touch display which is not over-sensitive tovibrational forces, and which prevents optical problems due to parallaxor internal reflections from arising. A rigid transparent faceplate withtouch sensitive elements thereon is mounted to the display screen usinga transparent, elastic, adhesive compound film which has a similarrefractive index to that of the display screen. Since the touch-plate issupported by the adhesive compound and not by the display'stouch-sensitive transducers, forces generated during assembly of thedisplay unit do not apply undesirable bias to the transducer array.

Known methods for bonding faceplates onto monitors involve positioningspacers at the edges of the ITC screen, or of the faceplate, offeringthe faceplate up to the ITC screen, and sealing the edge of thefaceplate to the ITC to provide a physically contained volume for theadhesive. The seal may have a plurality of pin-holes around itsperiphery. The face of the ITC is held vertical, oriented so that anopening in the seal is at the top edge. Epoxy resin, which is mixed andoutgassed, is then pumped into the opening. The resin is allowed to runout of the pin-holes while the contained volume is being filled, untilthe operator determines that the space between the ITC and the faceplateis satisfactorily filled. The pin-holes and the filling opening are thencovered and the resin is cured. It is generally necessary to trim excessresin from the edges of the assembly after curing. Example methods ofthe above type are described in relation to the lamination of atransparent safety panel to a CRT screen in U.S. Pat. No. 4,656,522.

SU-A-1446868 describes a method for bonding an anti-glare filter to aCRT in which the filter is positioned horizontally at the bottom of amould, with transparent spacers set at its corners. Resin is poured overthe filter and the CRT is then lowered into the mould to press againstthe resin. The mould walls provide peripheral containment of the resin.

Required is a more efficient method for bonding faceplates to VDUscreens which is in particular suitable for manufacture oftouch-sensitive displays having touch-plates adhered to a displayscreen. It is desired to increase the scope for automation over theexisting faceplate bonding processes (which are generally reliant onoperator judgement as noted above), and generally to increase the speedand reduce the cost of the process.

Accordingly, it is a first aspect of the present invention to provide amethod of attaching a transparent faceplate to a screen of a visualdisplay unit (VDU), by adhesion of opposed surfaces thereof, comprisingthe steps of:

dispensing a volume of a transparent adhesive material onto at least oneof said surfaces;

bringing said surfaces together in a controlled manner to cause theadhesive material to spread across said surfaces towards their edges tofill the gap therebetween;

selectively curing the adhesive material at the edges of the opposedsurfaces to secure the faceplate to the screen of the VDU; and

subsequently curing the remaining uncured adhesive material.

The selective curing of adhesive around the edges of the opposedsurfaces, without necessarily curing all of the adhesive layer at thisstage, represents a great reduction in the time required to fix afaceplate to a VDU screen. Forming a permanent cured adhesive seal,which does not allow leakage of adhesive, enables the visual displayunit and attached faceplate to be removed from any support tool thatholds them for the bonding operation. This may be after a much shortertime period than is possible with the known processes which require thefixing adhesive to be completely cured in a single operation. Thepresent invention thus enables the production tooling to be used for theattachment process of the next monitor (or batch of monitors) after areduced time period, reducing the overall production cycle time.

Preferably, the step of selectively curing the adhesive at the edges ofthe opposed surfaces comprises selectively curing the adhesive materialat positions around the edges progressively as the advancingadhesive-to-gas interface reaches the edges at each of these positions.Due to the viscosity and surface tension forces of the adhesivematerials which are suitable for this bonding (e.g. acrylic and epoxyresins), the advancing adhesive interface tends to form a bead at theedge of the faceplate. Selective curing is then used to cure this beadas it forms to produce a permanent bond between the opposed surfaces.The endpoint of the adhesive fill of the gap between the opposedsurfaces may be automatically determined if the selective curing isautomatically actuated when the adhesive reaches the edges.

The progressive selective curing as the adhesive reaches the edge of thefaceplate has the effect of preventing mess and wastage of adhesivematerial that occurs if the adhesive is allowed to overflow from theedges, and reduces the time required for the bonding process. Since thefaceplates and CRT screens are substantially rectangular rather thancircular, overflow is to be expected to occur from the mid-regions ofthe edges of the opposed surfaces before the adhesive reaches the cornerregions unless there is either selective curing of the adhesive materialas described above or physical containment thereof prior to curing, orthe viscosity and surface tension properties of the adhesive materialare very carefully selected.

It is preferred that the step of selectively curing the adhesive at theedges of the opposed surfaces is carried out in response to a signalfrom a visual detection means arranged to detect the approach of theadhesive-to-gas interface to the edge of the opposed surfaces.Alternatively or in combination with the use of a visual detectionsystem, the selective curing may involve masking the adhesive layer,other than a portion of the adhesive layer which is in the region of theedges of the opposed surfaces, from curing irradiation. This irradiationmay be ultraviolet electromagnetic radiation.

Alternatively, the adhesive material may be cured rapidly as it reachesthe edge of the faceplate by laser or other thermal radiation, or byconductive heating--for example, from a hot roller to which the adhesivedoes not adhere. Another alternative method of rapidly curing theadhesive material at the edges of the faceplate is to pre-coat the edgesof the faceplate and the display screen with a chemical curing agent,which causes local rapid curing once the adhesive material comes intocontact with it.

An alternative method, which is more applicable to a lower level ofautomation than is the detection-responsive curing, is to use the knownindustry techniques of applying a bead of material--e.g. a fast-setting,non-spreading adhesive material such as CIBA Araldite 2010 to provide aphysical barrier to the final adhesive material fill. (CIBA and aralditeare trade marks of Ciba-Geigy AG). Small exit tubes or holes are made inthe bead for the purpose of filling.

It is preferred that the method according to this aspect of theinvention includes the step of locating reference points on thefaceplate and on the VDU for precise relative positioning of the opposedsurfaces. This step is generally intended to comprise either preciselymeasuring reference points on the VDU and the faceplate or seating thesurfaces to be bonded at precise reference points inautomatically-positionable support tools. The located reference pointsenable the final position of the screen relative to the faceplate to bedetermined without the need for the positioning of physical spacersbetween the surfaces to be bonded. Alternatively, positioning of thesurfaces may involve placing spacers between the surfaces as is known inthe art, but the avoidance of the need to position spacers between thesurfaces is desirable to simplify the bonding process. Morespecifically, the use of spacers is undesirable in touch-sensitivedisplays, since their necessary rigidity constrains the freedom toprovide displays in which the faceplate is physically moved relative tothe VDU display screen in response to a touch stimulus. Spacers, even iftransparent, may also produce undesirable visible effects.

In a second aspect, the present invention provides a method of attachinga transparent faceplate to a screen of a visual display unit (VDU), byadhesion of opposed surfaces thereof, comprising the steps of:

dispensing a volume of a transparent, adhesive material onto at leastone of said surfaces;

bringing said surfaces together under the control of signals from adetection system to cause the adhesive material to spread across saidsurfaces towards their edges to fill the gap therebetween, the detectionsystem being arranged to detect the onset of entrapment of gas behindthe advancing adhesive-to-gas interface and to transmit control signalsto avoid such entrapment; and

curing the adhesive material to secure the faceplate to the screen.

The avoidance of air bubbles in the adhesive layer between the faceplateand the VDU screen is extremely important because of the undesirabilityof visible air-adhesive interfaces within this layer and of the visualeffects which will arise if the layer separating the faceplate and theVDU screen contains patches which have markedly different refractiveindexes. The adhesive material should generally be outgassed prior tothe step of dispensing adhesive onto a surface to be adhered, assumingthe adhesive material is such as to require such a process.

In a third aspect, the present invention provides a method of attachinga transparent faceplate to a screen of a VDU, by adhesion of opposedsurfaces thereof, comprising the steps of:

locating reference points on the faceplates and on the VDU for preciserelative positioning of said surfaces;

dispensing an adhesive material onto at least one of said surfaces;

bringing said surfaces together in a controlled manner, the finalposition of the screen relative to the faceplate being determined withreference to said reference points without the need for the positioningof physical spacers between said surfaces; and

curing the adhesive material to secure the faceplate to the screen.

The method of positioning the components which are to be securedtogether according to reference points located thereon avoids thegenerally accepted prior art requirement for spacers to be positionedbetween the surfaces, and thereby provides a means to improvemanufacturing throughput over these prior art methods. This isespecially useful in view of the desirability of increased automation ofmanufacture.

In a preferred embodiment, the invention provides a method of attachinga transparent plate to a screen of a visual display unit (VDU) for acomputer system, by adhesion of opposed surfaces thereof, which plate isto cooperate with touch sensing means for producing a plurality ofelectrical signals in response to and representative of a touch stimulusapplied to the plate by a user, for provision of a touch-sensitivedisplay, comprising the steps of:

locating reference points on the touchplate and on the VDU for preciserelative positioning of said surfaces;

dispensing a volume of an outgassed transparent, elastic, adhesivematerial onto one or both of said surfaces;

bringing said surfaces together in a controlled manner to cause theadhesive material to spread across said surfaces towards their edges toform an adhesive layer which fills the gap therebetween, the finalposition of the screen relative to the faceplate being determined withreference to said reference points without the need for the positioningof physical spacers between said surfaces;

selectively curing the adhesive at the edges of the opposed surfaces toform a bead of cured resin which secures the touchplate to the screen ofthe VDU; and

curing the remaining uncured adhesive material.

Embodiments of the invention will now be described in more detail, inorder that the invention may be more fully understood, with reference tothe accompanying drawings in which:

FIG. 1 is a block diagram of an example computer system including atouch-input display such as may be produced according to an embodimentof the present invention;

FIG. 2 is a schematic representation of a transducer arrangement whichmay be used to produce a touch-input display implementing the presentinvention;

FIG. 3 is a perspective view of a CRT assembly to which a touch-platehas been bonded according to an implementation of the present invention;

FIG. 4 is a flow diagram of the sequence of steps performed in theattachment of a faceplate to a visual display unit according to apreferred embodiment of the present invention;

FIG. 5 shows the arrangements of the various components of a bondingapparatus according to an embodiment of the invention;

FIG. 6 shows the arrangement of position-measurement devices for a CRT,which form part of the tooling for faceplate bonding according to anembodiment of the invention;

FIG. 7 shows resin dispersement between a faceplate and CRT according toan embodiment of the invention:

FIGS. 8a-8b show paths of a beam of light impinging of a faceplateseparated from a CRT by an air gap and resin, respectively, according toan embodiment of the invention;

FIG. 9 is a diagrammatic representation of a resin surface detectionsystem according to an embodiment of the invention:

FIG. 10 is a graph showing a profile of light measured by the detectorshown in FIG. 9 relative to the light profile on the faceplate accordingto an embodiment of the invention;

FIG. 11 schematically shows the effect of an angular position error ofthe faceplate according to an embodiment of the invention;

FIG. 12 is a diagrammatic representation of a resin surface detectionsystem having an array of sensors according to an embodiment of theinvention;

FIG. 13 shows a circuit diagram that determines use of a proper sensorfrom the sensor array of the detection system of FIG. 12 according to anembodiment of the invention;

FIG. 14 is a graph showing detection results using a detection systemaccording to an embodiment of the invention;

FIGS. 15a-15c show three stages of a process for bonding a faceplate toa CRT according to an embodiment of the invention;

FIG. 16 shows physical arrangement of seals in contact with faceplateedges according to an embodiment of the invention:

FIG. 17 shows a fan having a profiled nozzle for directing air onto aresin interface between a faceplate and a CRT according to an embodimentof the invention;

FIGS. 18a-18b show a system for implementing faceplate measurement tocalculate resin volume according to an embodiment of the invention;

FIG. 19a-19b show the effect of faceplate radius variations relative tothe CRT curvature radius, and a flat plane mapped error volumerepresentation according to an embodiment of the invention; and

FIG. 20 is a graph showing the effect of faceplate radius error on therequired resin volume according to an embodiment of the invention.

An example of a computer system including a touch-input display is shownschematically in FIG. 1. The computer system includes a centralprocessing unit (CPU) 1 for executing program instructions. A busarchitecture 2 provides a data communication path between the CPU andother components of the computer system. A read only memory 3 providessecure storage of data. A fast random access memory 4 provides temporarystorage of data. Data communication with a host computer system 5 isprovided by a communication adapter 6. An input/output adapter 7provides a means for communicating data both to and from a mass storagedevice 8.

A user operates the computer system using a keyboard 9 which isconnected to the bus architecture via a keyboard adapter 10. Atouch-input enabled display unit 11 of the present invention provides avisual output from the computer system. The visual output is generatedby a display adapter 12. The user can also operate the computer systemby applying touch stimulus to a touch-sensitive input screen on thedisplay unit. A touch-input screen adapter 13 connects signals from thetouch-input screen to the bus architecture of the computer system.

Referring now to FIG. 2, the touch-input screen has four force-sensitivetransducers 21,22,23,24 bonded proximate the four corners of asubstantially rectangular, transparent touch-plate 20. Four discreteelectrical signals 25,26,27,28 are generated by the four transducers,each signal being produced by a separate transducer. The four electricalsignals are processed by a signal conditioning and analogue to digital(A to D) converter portion 29 of the touch-input screen adapter 13. Thesignal conditioning and A to D conversion portion 29 then produces abinary data output 30 indicative of relative forces measured by the fourtransducers. The binary data output is therefore representative of alocation on the display at which a touch stimulus is applied.

Referring now to FIGS. 3 and 4, a touch-input enabled display unit has atouch-plate 20 bonded to the face of a CRT 40 by a uniform layer or film41 of a transparent, adhesive, elastic compound. A transducer array ismounted on the touch-plate 20, as indicated in FIG. 3 by transducers21,22,23,24 which produce respective electrical signals 25,28,27,26.

In an automated method of touch-input enabled display manufacture (asrepresented in FIG. 4 as a sequence of process steps; the apparatus usedin the process being shown schematically in FIGS. 5 and 6), a faceplate20 and an integrated tube component (ITC) 40 are each located 50 in arespective support tool 150,160 for bonding together. The first tool isa location plate 152 for the faceplate, which supports the faceplate ina horizontal position with its concave face upwards, by means ofpositioning pins 154. The second tool carries the ITC via lugs 42. Thelugs are not located in very precise positions (their positions maytypically vary by 2 mm in a direction perpendicular to the plane of thedisplay). Specific reference points for automatic positioning aretherefore located 60 on each of the faceplate and the ITC surface. Glasssurfaces such as CRT screens are conventionally specified by fourreference points often referred to as the Z points. They are typicallynear the screen edge along the diagonals, and locate the surface inspace. For one implementation of the reference point location, fourlocation probes 200 for the ITC and four (in the form of the supportpins 154) for the faceplate are set at the known Z point positions. Theprobes 200 for the ITC, which are spring loaded, are brought intocontact with the ITC surface to allow the vertical positions of thereference points to be determined, for example by a computer, byconnecting the probes for example to a linear potentiometer. Opticalsensors could be used as an alternative to the potentiometer connection.

The measurement of the ITC Z point positions uses three horizontallypositioned location pins 210 in addition to the aforementioned probes.The edges of the CRT screen are pushed against the pins to determine thelocation--two pins against one edge and one against a secondperpendicular edge. The arrangement of these measurement devices isshown schematically in FIG. 6 (with the position of the CRT duringlocation measurement being shown by broken lines). Location informationcan again be stored in a computer. The position of the front surface ofthe ITC is then precisely known, in three dimensions. The initialposition of the faceplate in its positioning tool is determined by itssupport pins 154. The positioning of these pins, which are small enoughnot to obscure electromagnetic radiation transmitted from beneath thefaceplate, is then further controlled (motor driven) to allow alignmentwith the plane of the CRT screen after its Z point measurement, withoutthe need for physical spacers to be positioned between the surfaces. Thefaceplate support tool also uses three horizontally positioned locationpins to fully determine its position. The distance between the faceplateand the CRT screen is now known.

Whilst the embodiment of the invention described above uses threedimensional positioning measurement for both the faceplate and the CRT,and then adjusts the position of the faceplate to provide correctalignment, alternative embodiments could equally provide for adjustmentof the positioning of both components or of the CRT only.

The front surface of the ITC is then wetted 70 with a dilute solution ofthe adhesive material in a solvent, and the solvent is allowed toevaporate. This guarantees that the surface of the ITC, which willgenerally be textured so as to reduce reflections therefrom, iscompletely penetrated by adhesive and no air bubbles are entrapped. Ifthe wetting characteristics and viscosity of the base are optimised,this additional wetting step is not required.

A measured volume of outgassed epoxy or acrylic resin is dispensed 80onto the centre of the faceplate. A UV-curable adhesive which may beused is CIBA Araldite 4001 VV or Loctite 350 (Loctite is a trademark ofLoctite Corporation). Other adhesive materials could be used asalternatives. The resin compound (or other adhesive material) desirablyhas a similar refractive index to the two layers which it bondstogether, but the refractive index of the textured coating on the ITCfront surface is generally different from that of the polished rearsurface of the faceplate. Minimising reflections from each of theglass-adhesive interfaces thus requires the adhesive material to have arefractive index which is either a compromise between these two glasssurfaces or is not constant. The acrylic or epoxy resin may be selectedto chemically soften and swell the silica/polymer textured coating whichis known to be provided on a CRT screen to reduce surface reflections.This chemical change has the effect of producing a gradual rather thanan abrupt change in refractive index at the interface and thus furtherminimises reflections.

The ITC is then lowered 90 at a controlled rate towards the faceplate,under the control of servo-control signals from a movement controller164. As the rate of lowering is increased, so is the tendency to entrapair behind the advancing adhesive-to-air interface. Thus, the rate oflowering is maintained at a rate which will avoid the entrapment. Thelowering speed may be automatically controlled in response to signalsfrom a visual detection system, which uses a television system 156,158(described in more detail below). In an alternative embodiment, the ITCis manually lowered into position. As this lowering operation iscontinued, the approaching surfaces of the ITC and the faceplate forcethe resin 170 to spread out laterally from the centre to the edges ofthe plate, filling the gap between the ITC and the faceplate.

As the resin is forced to spread across the opposed surfaces towards theedge of the faceplate, its position is detected by a visual detectionsystem. The detection system may comprise a television camera 156 whichviews the faceplate through the support plate 152 of the support tool150. The underside of the faceplate is illuminated by visible light froma light source 158, and the camera captures images which are then sentto a capture frame store in a computer 162. Signal processing isperformed to identify the position and speed of the air-to-resininterface at different times using identification of changes ofrefractive index. Typically, contrast and edge enhancement techniquescommon in optical signal processing will be used. One suitable visualdetection system is the Synoptics Synapse system with the Semper 6Plusimaging language (Synoptics and SEMPER are trademarks of SynopticsLimited).

Such a detection system, and an alternative detection system, which issuitable for use in detecting the resin meniscus position forrefractive-index-matched resin, are described in detail below.

As the resin reaches the edge of the faceplate, a signal is transmittedfrom the detection system to a UV curing apparatus 166. This signalactuates irradiation (100) of the edge of the assembly with ultravioletelectromagnetic radiation, by controlling shutters in front of the UVlight sources, to cure the resin at the periphery of the faceplate andthereby to permanently bond the faceplate to the ITC. The faceplatesupport tool must be optically clear to UV radiation. UV radiationsources are commonly used in industrial processes, and for thisapplication can be selected to optimise wavelength to the particularresin actuator. Such a source of radiation is the Loctite UVAloc 1000.The signal actuating the irradiation apparatus may be generated at theinstant that the advancing resin interface reaches the edge of thefaceplate at any position (and then progressive selective curing and ITClowering may be performed simultaneously until the resin has reached allpoints around the periphery of the faceplate). Alternative methods ofcuring at the periphery of the faceplate include use of a thermo-settingresin and the application of heat (e.g. by thermal radiation orconductive heating).

The (vertical) gap between the faceplate and the ITC may bepredetermined by the known Z point measurement, but preferably thevisual detector determines when the resin has reached all points of theperiphery of the faceplate and then the lowering is automaticallystopped. Thus, the reference points on the faceplate and on the ITC areused to set the horizontal alignment and the visual sensor is used todetermine the end point of the ITC's lowering movement. The vertical gapis thus adapted to any mechanical tolerance variations of the faceplateor the ITC screen, and there is no need for physical spacers.

The assembly is then removed 110 from the tool. The remaining uncuredresin, if any, is then cured 120 by additional ultravioletelectromagnetic radiation through the faceplate. Alternatively, theremaining uncured resin is cured by infrared lamps or conductionheating.

A method of attachment of a faceplate to a ITC screen of a CRT monitorfor the manufacture of a touch-sensitive display has now been describedby way of an example implementation of the present invention. It willhowever be appreciated that the invention is also applicable to theattachment of other faceplates such as anti-reflective screens, and thatthe invention may use a different display device such as a liquidcrystal display panel or a gas plasma panel. Additionally, the method ofthe invention has been described as a stage in the production of adisplay unit but could equally be performed as a method of retrofittingtouch panels to assembled monitors. In each of these methods,touch-input enabled display units can be produced using standardmanufacturing processes developed for non-touch-input displays, with theadditional step of attachment of a touch-plate and sensors. Retrofit isnot the preferred method of attachment in view of difficulties inperforming such an operation without damaging the monitor.

An alternative to the rapid curing of the adhesive material at the edgesof the faceplate is to use physical containment of the adhesivematerial. In one such process, an elastic gasket is automaticallypositioned on the upper surface of the faceplate. The gasket is of sucha thickness that it forms a seal between the CRT and the faceplatebefore the advancing adhesive interface reaches it. The gasket has aplurality of holes pierced through it, at the positions (proximate thecorners of the faceplate) which are the last to be reached by theadhesive material as it is forced to spread outwardly between theopposed surfaces, to allow escape of air. The holes are manually orautomatically plugged once reached by the advancing adhesive. Theaspects of the present invention of using automatic detection of theonset of the condition of gas entrapment and of the adhesive reachingthe edge of the assembly (and automatic control in response to thedetection) are equally applicable to a method which uses physicalcontainment of the adhesive as an alternative to rapid selective cure.

It should also be noted that the selective curing aspects of theinvention are applicable to manual as well as automated attachmentprocesses.

As discussed earlier, a digital image capture and processing system maybe used to track the travel of the resin and to identify processanomalies such as bubbles or debris that may accidentally gain entry tothe dispensed resin. This computer controlled technique is sensitiveenough to observe the resin accurately, determine its location, anddigitally communicate the results to an automated controller. However,the measurement is highly sensitive to the illumination conditions, andso only certain illumination configurations are suitable to theimplementation of a vision system in a practical manufacturingenvironment.

Experimentation showed that some form of active illumination must beused to allow the vision system to be employed successfully.

From the experimentation, it was observed that the most importantcomponent of the ambient light required to enhance the meniscus locationwas that from the side of the ITC fixture. It is believed that light isguided towards the meniscus by total internal reflection from the ITCand touchplate surfaces. At the meniscus this light is reflected, hencethe observation of lower intensity inside the meniscus and a highmeasurement outside. Therefore the introduction of active side lightingis the optimum solution, maximising detection efficiency. The followingare examples of how active side lighting could be applied. Firstly,baffled light tubes can be positioned to direct light at an angle nearthe edge of the faceplate. If edge seals are present and only cornergaps are available to project light through, projector bulbs withreflectors could be positioned to direct light into the corner gaps. Thelight could be directed so as to reflect along the glass by internalreflection and so diffuse towards the meniscus edge.

The digital vision system used for detection of light reflected from theresin interface is now described. Semper 6 Plus software was used inconjunction with a Synapse framestore. Semper 6 Plus software fromSynoptics is a general purpose image processing language. It gives acomplete toolkit of image processing functions that include thefollowing:

1. Image processing, display and control.

2. Image analysis and transformations.

3. Geometric and arithmetic operations on images.

4. Image file management.

5. Framegrab--takes images from a camera or microscope.

6. Stores one or more image frames in memory.

7. Sends a frame to a television monitor to display an image.

8. Allows customised, automated processing of images.

A perpendicular line from the centre of the meniscus to a corner of thetouch screen was set so that the images could be analysed along the sameradial direction. For each acquired image a small area was marked andthen processed to calculate the average pixel intensity over that areaas well as the standard deviation of pixel intensities. This wasrepeated along the pre-determined line. The values for mean areaintensity and standard deviation can then be graphed against radialdisplacement. Initial experiments highlighted the need to vary the sizeof the sample area to optimise the signal to noise ratio for the pixelintensities and the lateral resolution. From the results obtained theoptimum sample area was chosen as 1.25×1.25 mmsq. (5×5 pixels).

METHOD 1: SINGLE IMAGE-MULTIPLE SAMPLE AREA ANALYSIS

A set of images were acquired with the meniscus at different radialpositions. Each image was analysed using the same method. This invovledlocating the intersection of the sampling line and the meniscus, thentaking ten samples either side of this point along the sampling line. Aplot of mean area intensity versus radial displacement was constructed.The standard deviation of the pixel intensities in the sample area wasalso graphed against radial displacement. Both these measurements gavewitness to the location of the meniscus on each image. A very distinctpeak was found for standard deviation verses radial displacement atapproximately 20 mm of radial displacement. This peak identified theexact location of the resin meniscus.

METHOD 2: MULTIPLE IMAGE-FIXED SAMPLE AREA ANALYSIS

For this analysis a sampling position was located and fixed.Measurements of the mean pixel intensity and standard deviation werethen taken from this area for a set of images captured as the meniscusmoved through this point to determine the feasibility of using intensityand standard deviation as flags for meniscus monitoring at the faceplateedge.

Image obtained during experimentation were analysed to produce a graph.In particular, image toward the end of the process (ie. the edge of thefaceplate) must be analysed for it to be effective. 2 indicators can beused to detect when the resin meniscus has reached the fixed samplingpoint: the means pixel intensity change and the standard deviation.

Multiple Image Fixed Sample Area Analysis sis merited to be the betterof the described image analysis techniques as the changes in pixelintensity and standard deviation are more distinctive. With the visionsystem a variety of fixed sampling areas can be monitored to furtherimprove the meniscus tracking. Additionally, it is necessary to checkfor a defect free bond and in all other bonding processes this check isdone by human visual inspection, which is always time consuming andcostly. The vision system is therefore used, increasing the automationof the process. At the point where the ITC panel and resin first meet asthe panel is being lowered air bubbles can be created. The vision systemwas given the task of detecting the presence of bubble formation ordebris so that the process can be stopped and the ITC and touchscreensalvaged.

A method and system for detecting the position of the air-to-resininterface as liquid resin is squeezed between the opposing surfaces ofan ITC screen and a faceplate was described earlier. An alternativeautomatic detection system will now be described. FIG. 7 shows afaceplate 20 held beneath a CRT 40 with a small separation gap, in theprocess of bringing the surfaces of the CRT and faceplate together afterresin 41 has been dispensed onto one of the surfaces. The detectionsystem now to be described is equally applicable to methods of faceplatebonding in which the relative positions of the faceplate and CRT are setand then the gap between them is subsequently filled with resin. Inorder to determine when to initiate certain actions in the bondingprocess (e.g. initiating edge sealing), it is necessary to be able tosense the position of the resin meniscus or at least to detect when theliquid resin reaches key points over the surface of the faceplate. Thisis implemented by arranging for detection at one point near each edgeand also near each corner--eight sensing positions in total.

FIGS. 8a and 8b give simplified representations of what happens when abeam of light 300 is focused onto the surfaces, firstly with an air gap310 between the faceplate and the CRT in FIG. 8a, and secondly with arefractive index matched resin 41 filling the gap in FIG. 8b.

With reference to FIG. 8a, an incoming beam of light strikes thefaceplate and approximately 4% of this light is reflected because of thedifference in refractive index between air and glass. Since thefaceplate surface is slightly roughened, the reflected light is partlyspecular and partly diffuse. The ratio of specular to diffuse reflectedlight is approximately one to one for commonly used faceplate materials,and therefore 2% of the collimated light continues as collimated beam A.96% of the light goes through the faceplate and strikes its rear surfacewhere 4% is reflected. This 4% strikes the front (roughened) surface and2% emerges as collimated beam B. 96% of the light at the rear faceplatesurface emerges and continues until it strikes the roughened CRTsurface, where we get reflection in the same way as before. Thereflected light again passes through the roughened faceplate surface toemerge as collimated beam C.

The total collimated light that can be collected is A+B+C, and isapproximately: ##STR1##

In the resin filled case of FIG. 8b, light is reflected from the frontsurface as before, but both the rear faceplate surface and the CRT frontsurface are now covered with index matched liquid so that no significantreflection takes place.

Thus light beam E has magnitude: 0.04×0.5=0.02

The change ratio in the light that can be detected as the meniscus movesthrough the path of the light beam ban be seen to be:

0.0484/0.02=2.4

In practice, it is also possible to detect disturbances as the curvedmeniscus passes the light beam, with various reflecting angles and rapidchanges in these, depending on the size of the light beam relative tothe gap width, and the angle of the light beam. Typically the touchplatethickness will be 2 mm and the gap width 1 to 2 mm.

The light which enters the CRT glass screen will not produce significantfurther reflections, since it will be double attenuated by the CRT's 50%neutral density filtering, as is known in the art.

FIG. 9 is a diagrammatic representation of a system developed for resinsurface detection. A 0.3 mm pinhole 320 was positioned close to ahalogen lamp 330, and the light collected with a 35 mm focal length lens340. This focused light passing through the pinhole onto the surface ofthe faceplate 20 positioned 185 mm away. A light dependent resistor 350of diameter 3 mm was placed 250 mm away from the plate, with a halfangle of 26 degrees.

Because the light is focused onto the plate surface, the reflected lightwill diverge, and from a flat plate the 1 mm spot will increase to 2 cmdiameter. However, the faceplate is actually curved (eg radius 580 mm ona 14" CRT, and 1200 mm on a 15" FST), and therefore the light beamdiverges, even more, to 4 cm as measured.

These two cases are shown as the different diameter reflected beams inFIG. 9.

In FIG. 10 is shown the results of measuring the spot profiles at thedetector, relative to the spot profile on the plate surface. A furtherproblem is to counter the effect of tolerances in plate angle andcurvature (eg as shown in FIG. 11), so that light could always beguaranteed to strike the detector. Shown on FIG. 10 is the shift (x)that occurs if the plate angle changes by one degree. In the case of aflat plate (eg the vertical axis of a Sony Trinitron or MitsubishiDiamondtron CRT), or in the case where a smaller spot was used (e.g.where the source of illumination was a laser) or where tolerancesexceeded one degree, it can be seen that light beam/detector coincidencemight become marginal. The effect of an angular position error of thefaceplate is represented schematically in FIG. 11.

FIG. 12 shows a solution to the tolerance problem. Here, an array 360 ofsensors (350, 350', 350") is used, the particular implementation being arectangular 3×3 array of low cost light-sensitive resistors orphotodiodes. With this arrangement one sensor in the array will alwaysbe substantially illuminated, irrespective of the angular tolerance ofthe plate.

It is then necessary to determine which sensor to use in the detection(simply placing all sensors in series or parallel would reducesensitivity), and a method of doing this is shown in FIG. 13. Thesensors are taken to a 9 channel electronic switch 370 and the firstsensor 350 in the array is selected and its output stored in a sampleand hold circuit 380. The next sensor 350' is now selected and comparedto the previous value in the sample and hold circuit. Depending on whichis larger, the sample and hold is either left unchanged or is updated bythe new sensor. The sensor number will be stored. Cycling is carried outto find the number of the sensor with the largest output, and this willbe the sensor used. This selection process is performed as each newfaceplate is placed in position. The switch output is passed to an A/Dinput on a computer, and the process is then controlled by computer.

The detection results shown in FIG. 14 were produced using the simpleand low cost system described above. The graph shows the change inresistance of the light sensor as the liquid interface passes throughthe light beam, for different spot diameters and angles of incidentlight. Best results were obtained with a small spot diameter of 1 mm anda light beam angle of 26 degrees to the touchplate. An enhancedresistance peak was detected as the meniscus passed through the beam.The narrow collimated beam from a laser diode could produce furtherimprovements to detection ratios. The light detector was found to worksatisfactorily in low ambient light if appropriate shielding is used forthe detector element.

Described earlier are methods of selective curing of resin around theedges of a faceplate, and alternatives to selective curing that employphysical containment of resin. We will now describe implementation ofsuch physical containment methods in more detail, describing a firstimplementation using flexible and movable seals, and a secondimplementation using an "air knife" containment. These containmentmethods involve the following common sequence of steps:

1. bringing the opposed surfaces of the faceplate and VDU together untilthe adhesive material has been displaced sufficiently towards the outeredges of the surfaces for it to be detected, by a detection meansarranged to detect the adhesive material, at a position proximate anouter edge of the opposed surfaces;

2. responsive to detection by the detection means of the adhesivematerial at a position proximate one of the outer edges, actuating acontainment mechanism to prevent leakage of the adhesive material fromthe outer edges;

3. bringing the opposed surfaces closer together until the adhesivematerial has been displaced sufficiently for it to be detected at aposition proximate one of the four corners of the opposed surfaces;

4. responsive to detection by a detection means of the adhesive materialat a position proximate one of said corners, actuating a curingmechanism to cure only the adhesive material which is proximate said onecorner;

5. bringing the opposed surfaces together and actuating a curingmechanism to cure the adhesive material proximate each of the remainingcorners of said surfaces in turn, in response to detection of theadhesive material at respective positions proximate each corner; and

6. curing the remaining adhesive material subsequent to the adhesivematerial reaching all of said corners.

Methods of bonding have been described in which liquid resin isdispensed onto the centre of a surface of a faceplate, and the ITC isthen lowered towards the faceplate resulting in displacement of theresin. As the resin reaches the edges of the faceplate it is necessaryto provide a seal and, since the resin will progressively advance alongthe edges until it reaches the corners of the faceplate, the seal mustalso accommodate this. The resin is cured to permanently bond thefaceplate to the ITC screen. The three stages of this overall processare represented in FIGS. 15a, 15b and 15c respectively, with physicalcontainment seals 400 being shown in each of FIGS. 15b and 15c.Ultraviolet irradiation of the structure for curing purposes isrepresented in FIG. 15c by arrows 410.

The seals are typically formed of resilient silicone, moulded to conformto the ITC and touchplate profile. These seals are pressed against eachof the four edges of the faceplate and ITC screen, but with each cornerleft open to allow air to be expelled. Typical silicone mouldingmaterials are sufficiently flexible to permit the few millimeters (atmost) of movement of the ITC towards the faceplate after the seals arebrought into position (i.e. the movement between the stages representedby FIGS. 15b and 15c).

The aforementioned resin meniscus edge sensors are positioned at each ofthe four corners to detect when the resin reaches that point. Then a UVlight is turned on (or a shutter opened), collimated just to irradiatethat corner. By choice of the light power, the setting time can becontrolled, and a relatively low power is used so that a thin skin onlyis formed during the available few seconds before all corners arereached. This thin skin seals the corner and prevents resin escaping,but does not prevent the fraction of a millimeter movement of ITC totouchplate that occurs subsequent to its formation.

The total sequence of events is:

1. Begin the resin displacement operation.

2. Determine when resin gets to within 2 cm of any edge using a lightdetector or a vision system.

3. Halt resin displacement by stopping the descent of the ITC.

4. Bring in the edge seals with a slight pressure (a pressure equivalentto finger pressure is generally acceptable).

5. Continue resin displacement.

6. Detect when resin reaches each corner and illuminate just the cornerwith low power UV light (as shown in FIG. 16).

7. When the last corner is reached, stop resin displacement.

8. Turn on high power UV lights to irradiate the whole front surface ofthe ITC/faceplate, and so permanently cure the resin.

At a distance from the edge of 2 cm, the remaining vertical distance forthe ITC to be lowered is approximately 2.5 mm, and the seals aresufficiently flexible to accommodate this. The most suitable seals areformed of very flexible Silicone rubber with a built in release agentthat avoids the need to separately coat the seals, as is known in theart.

FIG. 16 shows the physical arrangement of silicone rubber edge seals 400in contact with the edges of a faceplate 20. Four collimated low-levelUV light sources 116 are positioned proximate the corners of thefaceplate for irradiating the resin when it is detected by therespective sensor for that corner position.

The method described above also works with cylindrical ITCs such as theSony Trinitron or Mitsubishi Diamondtron, except that seals are put intoplace immediately on two of the edges to act as a barrier to thedispensed resin.

A further alternative to the above-described use of flexible seals forthe containment of resin during faceplate positioning will now bedescribed. Mechanical seals are avoided by use of an air barrier to sealthe edges of the assembly while the resin is mobile. The air barrieraccommodates the progressive advance of the resin along the faceplateedges until it reaches the corners. FIG. 17 shows a fan 420 having aprofiled nozzle attached thereto, which is arranged to direct air ontothe resin interface 41 between the ITC 40 and faceplate 20.

As the adhesive moves to the edge of the faceplate, air pressure is usedto halt and maintain the advancing adhesive, providing an air barrier or"control knife" that achieves and maintains the required deformation ofthe resin meniscus to prevent resin leakage. As the adhesive proceeds tofill the gap between the surfaces up to the corner any interfacial airescapes through the open corners whilst the air barrier confines theadhesive within the faceplate boundary.

A static air barrier implementation was observed to require progressivemovement along the edge of the interface as the resin moved outwards tothe corners of the faceplate. The static system is improved upon byusing a resin detection or vision system to provide observationalfeedback with which the operation parameters of the air barrier can becontrolled. These parameters include the shape of the nozzle, theposition of the fan/nozzle, and the power of the air barrier. Thisdynamic system supports a repeatable and automated dispensing andcontainment of the adhesive between the ITC and faceplate. The air mustbe dust free to prevent contamination of the resin optical bond, and soa filter 425 is incorporated in the fan. The air flow is keptsufficiently stable to minimize flow or stress during the curing phase.

With this method of resin dispensing and containment there is theproblem of tolerance errors on the faceplate radius and therepeatability of the geometrical alignment of individual CRTs andfaceplates. As a result, the adhesive resin cannot be guaranteed tosimultaneously reach the 4 corners at the same time, hence a means ofsealing the individual corners is required to make this method ofsealing practical. The same curing arrangement described previously forflexible physical containment seals is used, with resin meniscus edgesensors positioned at each of the four corners. A UV light is turned on,collimated just to irradiate that corner, forming a thin skin during theavailable few seconds before all corners are reached. Following this,higher power UV lights are used to irradiate the whole front surface ofthe assembly to permanently cure the resin.

The advantage of the described methods maintaining the resin in a liquidstate until the final cure inhibits stress growth as compared with themethods of bonding in which selective resin curing is carried out aroundthe whole edge of the faceplate as a first step followed by a subsequentstep of curing the remaining resin. Implementations which use an airbarrier prevent resin overflow without mechanical sealing, therebyminimising the amount of disassembly required after bonding and hencereducing both the time required for the total manufacturing process andthe likelihood of damaging the assemblies. The absence of physicalbarriers aids stress free curing.

An optimisation for automated implementations of the panel bondingprocess involves measurement of each faceplate prior to bonding andcontrol of the amount of resin material used for bonding in dependenceon this measurement. In automated volume production processes, the costof materials can be the predominant manufacturing cost for a monitor andthe cost of resin used for faceplate bonding can be a significantelement of that material cost. Clearly the volume of resin that isrequired for each monitor is dependent on the separation distancebetween the faceplate and the ITC screen, and so a saving in cost isachieved by a minimisation of the separation distance and identificationof the minimum volume of resin needed to fill the gap. A problem ariseswhen seeking to minimize the volume of resin used, due to the variationsbetween the physical dimensions of individual faceplates--as with anymanufacturing process, the manufacture of faceplates produces faceplateswith dimensions (notably the radius of curvature) that vary withintolerance limits. The variations between individual CRT screens willgenerally be much smaller than those between faceplates, in view of thehigh precision processes used in CRT production. If the same fixedvolume of resin is to be used for each CRT-touchplate combination, thenthe volume must be sufficient to cope with the worst case tolerancesituation or incomplete filling of the gap could result.

The solution to the above problem of how to minimise resin usage is tooptimise the volume used for each faceplate in turn. This is implementedby measuring the radius of curvature of each faceplate prior to bonding.The output of this measurement is passed to a control computer and usedto calculate a deviation from the nominal radius. The optimum volume ofresin is then determined according to a volume determination functionstored in the control computer's memory. The control computer is linkedto the resin dispensing system to control dispensing of this calculatedoptimimum volume. This optimisation of resin volume use is made possibleby the avoidance of dependency on fixed spacers for determining spacingbetween the CRT and the faceplate, which avoidance is a feature of thevisual-detector-controlled automated positioning of faceplates describedearlier. The measurement data for individual faceplates can also be usedin the control of the faceplate production process.

FIGS. 18a and 18b show a system for implementing faceplate measurementfor the calculation of resin volumes. A plurality of measurement pins450 are fitted to the touchplate tool support plate 152 in a matrixarrangement. The measuring pins are spring loaded and connected tolinear potentiometers 460 which are multiplexed to a computer 164. Othersimilar readout devices could equally be used. Symbol 470 in FIG. 18arepresents the reference plane of the support plate. From the matrix ofmeasurement points, the computer can estimate an area profile for thepanel. FIG. 19a shows a representative example of the effect of radiusvariations, in this instance for a faceplate 20 having a larger radiusof curvature than the CRT 40 such that the gap between them is largeradjacent their edges. The "error volume" between the CRT and thefaceplate (i.e. that part of the volume spacing which results fromvariations from the nominal dimensions for faceplates) may bemathematically mapped onto a flat plane for reference, as representedand exemplified in FIG. 19b. The inclusion in this figure of gapdimensions for a faceplate with edge dimensions 300 mm by 220 mm arealso merely exemplary. The resin volume estimation uses well knownintegration techniques, with a predefined minimum gap (e.g. 0.5 mm)which is to be maintained between CRT and faceplate at any point duringbonding.

A graphical construction may be used as an alternative to integration. Agraph of resin volume (V) against the corner spacing tolerance (d)resulting from variations in faceplate radii of curvature, again with aminimum gap of 0.5 mm at the centre, is shown in FIG. 20. While onlyexemplary, FIG. 20 shows that faceplate radius error can have a largeeffect on the required resin volume, and so the potential materialsavings of the volume optimisation are large unless faceplatemanufacture is sufficiently precise that dimensional tolerances are verylow.

The preferred embodiment of the invention thus includes the step ofmeasuring faceplate dimensions for each faceplate and calculating fromsaid dimensions an optimum volume of adhesive material to be dispensedonto said at least one surface for forming said adhesive layer, the stepof dispensing then being controlled in dependence on said calculation.

We claim:
 1. A method of attaching a transparent faceplate (20) to ascreen of a visual display unit (40), by adhesion of opposed surfacesthereof, comprising the steps of:locating reference points on thefaceplate (20) and on the visual display unit for precise relativepositioning of said surfaces by a positioning tool without the need forthe positioning of physical spacers between said surfaces; dispensing(80) an adhesive material onto at least one of said surfaces; bringingsaid surfaces together (90) in a controlled manner using said referencepoints, such that the opposed surfaces displace the adhesive materialoutwards towards the edges of said surfaces to form an adhesive layerwhich fills the gap therebetween, wherein a termination point for thestep of bringing said surfaces together is determined using a detectionmeans for determining when the adhesive material has reached predefinedpoints proximate the outer edges of the opposed surfaces; curing(100,120) the adhesive material to secure the faceplate to the screen.2. A method according to claim 1, including the step of measuringfaceplate dimensions for each faceplate and calculating from saiddimensions an optimum volume of adhesive material to be dispensed ontosaid at least one surface for forming said adhesive layer, the step ofdispensing then being controlled in dependence on said calculation.
 3. Amethod according to claim 1, wherein said predefined points are locatedproximate each of the four corners of the opposed surfaces, saiddetection means then effectively determining when the adhesive materialhas reached all points around the outer edges of the opposed surfaces.4. A method according to claim 3, including the step of measuringfaceplate dimensions for each faceplate and calculating from saiddimensions an optimum volume of adhesive material to be dispensed ontosaid at least one surface for forming said calculation.
 5. A methodaccording to claim 1, including the step of measuring faceplatedimensions for each faceplate and calculating from said dimensions anoptimum volume of adhesive material to be dispensed onto said at leastone surface for forming said adhesive layer, the step of dispensing thenbeing controlled in dependence on said calculation.